This invention relates to a liquid developer for electrophotography and more particularly it relates to a liquid developer for electrostatic photography which is mainly composed of resin particles and is superior in negative charge stability and dispersion stability.
Conventional liquid developers used in the development of electrostatic charge patterns usually comprise a highly insulating carrier liquid in which are dispersed toner particles comprising a pigment such as carbon black and a resin such as an alkyd resin which imparts dispersibility and fixability. In general, these developers are not sufficient in charge characteristics or lack charge stability and hence a charge control agent such as calcium alkylsulfonate is often included.
However, these conventional liquid developers are still insufficient in charge characteristics and can afford sufficient image density only for high potential latent images. Especially, they are insufficient in charge stability and the charge characteristics may gradually change during storage for extended periods, resulting in a reduction of image density. Furthermore, their low dispersion stability causes agglomeration or settling when left to stand for a long period of time.
Further, these developers suffer from the disadvantage that offset printing plates made using these developers are insufficient in grease receptivity because of high pigment content.
Japanese Patent Unexamined Publication (Kokai) Nos. 52588/78, 54029/78 and 48738/82 disclose developers mainly composed of resin particles, which somehow overcome the above defects such as low dispersion stability, deterioration of properties with time and poor grease receptivity. However, when the resin particles disclosed in these publications are charged using charge control agents such as lecithin, calcium alkylsulfonates and polyamide resins, there often occur the problems that the particles are not charged at all, charge stability deteriorates with time and dispersion stability of the particles per se is lowered.